1. Field of the Invention:
This invention relates to electro-optical techniques for electronically sensing the motion of interference fringes and particularly to a technique for converting a signal expressed as a spatial distribution of light intensity to a time dependent function.
2. Prior Art and Information Disclosure:
A number of methods are used for real time processing of interference light signals that produce fringe patterns.
It is well known to use polarization effects, modulation or heterodyning in laser interferometers for position measurement or in laser based optical linear or rotary encoders. The objective is to provide high speed, digital, directional counts that span more than one fringe with not more than one fringe resolution.
Generating quadrature signals using polarization is one method where different polarization states are manipulated such that two photo signals with a 90 degree phase difference are constructed. Here each beam is spatially integrated and two separate detectors are used to yield the two signals. This method requires several precision optical elements.
According to the phase or frequency technique, one beam path is modulated mechanically or electro-optically with respect to the other and the modulation in the interference pattern is processed. Only one photocell is required and polarization and polarization optics is not required but optical modulation must be achieved.
Optical heterodyning involves interfering two beams of slightly differing frequencies. The resultant signal as detected by one photoconductor will include an AC signal at the difference frequency. One can directionaly compare the phase of this signal with a reference, i.e., the signal used to split the laser frequency.
Directional counting involves at least two signals. With polarization, separate polarization states and two detectors are used. The other techniques mentioned use one spatially integrated photosignal but involve two separate frequencies.
U.S. Pat. No. 5,018,862 discloses a dual detector providing quadrature signals by aligning the detector elements with an interference fringe within a laser beam. A portion of the beam is spatially integrated to produce one electrical signal and another portion produces another signal in quadrature.
U.S. Pat. No. 4,990,765 discloses elements of a serial diode array purposely fed a linear current ramp allowing a means for addressing individual pixels U.S. Pat. No. 5,266,796 discloses an absolute rotary encoder wherein multiple beams diffracted from a rotating grating impinge on an array and grating angle is deduced from beam locations.
U.S. Pat. No. 4,884,697 discloses an interferogram imaged on a CCD with pixel data being fit to sinusoids and non sinusoids to give fringe positions. This is a computer based approach intended for static optics testing. It is not useful for measuring rapid fringe motions.
U.S. Pat. No. 4,982,080 discloses individual detector cell currents individually converted to frequencies all on one substrate. The single line device output is fed to a spectrum analyzer to address the cell elements.
U.S. Pat. No. 4,169,980 discloses a modulation method of analyzing an optical fringe pattern. The first and second derivatives of the signal from a single photocell are used to define the fringe center.
U.S. Pat. No. 4,884,697 discloses using both polarization and active optical phase shifting to sense displacement with an interferometer using two separate photocells.
U.S. Pat. No. 5,165,045 to Eselun discloses a collimated beam diffracted to produce a standing wavefront along a surface coincident with a surface that emits a signal in response to changes in intensity along the wavefront.